This invention relates to composite blades for use in fluid flow machines and, more particularly, to increasing the strength of composite blade dovetails.
For many years attempts have been made to replace the relatively heavy, homogeneous blades and vanes of fluid flow machines such as gas turbine engine compressors with lighter composite materials. The primary effort in this direction has been toward the use of high-strength, elongated filaments composited in a lightweight matrix. Early work involved glass fibers, and more recent efforts have been directed toward the utilization of boron, graphite and other synthetic filaments. These later materials have extemely high strength characteristics as well as high moduli of elasticity which contribute to the necessary stiffness of the compressor blades and vanes.
Many problems have confronted the efforts to utilize these filaments, particularly in adapting their unidirectional strength characteristics to a multidirectional stress field. To a large extent, these problems have been overcome and composite blades have been demonstrated with performance characteristics, in many areas, equal to or better than their homogeneous metal counterparts, in addition to providing the expected and significant weight reduction.
However, at least one difficult problem remains to be solved: that being the design of a suitable connection capable of transmitting the airfoil gas, centrifugal and impact loads to a rotatable hub or disc. Compressor blade dovetail attachments presently represent the most expeditious and reliable method of incorporating the airfoils into a rotatable hub. For composites, this creates difficulty in that composite filament structures are least effective at fiber transitions or edges. Due to the difference in geometry between the airfoil portion of a blade and its dovetail, it becomes necessary to splay the individual filament laminates to shape the dovetail, and fill the voids therebetween with a filler material to provide a dense, load-carrying structure. Typically, the filler material comprises solid metallic inserts formed of titanium alloys, for example. This structure has proven ineffective for several reasons, not the least of which being the difficulty of obtaining an adequate bond between the metallic inserts and the metallic composite material (i.e., boron filaments disposed in an aluminum matrix) during the diffusion bonding process.
The very nature of composite materials is that they exhibit anisotropy such that the filament laminates do not conform to the shape of the inserts during the blade forming/bonding process. Poor conformance results in poor bonding. As the bonding pressure is increased, the inability of the metallic inserts and the boron filaments to deform as readily as the metallic matrix at the bonding temperature causes the boron filaments to be misaligned, crushed and fractured, thereby weakening the dovetail load-carrying capability. Furthermore, increasing bonding temperature produces high thermal residual stresses because of the different thermal expansion coefficients of the constituent materials.
Whereas the foregoing deficiencies relate to interlaminate inserts, the same may be said of dovetail outserts, those metallic sheaths disposed at least partially around the dovetail to provide filament laminate protection at the dovetail/hub slot interface and which effect a more uniform load transfer from the disc to the airfoil.
Thus, it becomes necessary to overcome the anisotropic behavior of the composite material and to provide for a more gradual transition between the composite materials and the homogeneous metallic inserts in order to have the dovetail and the slots remain a viable means of attaching a composite blade to a rotatable hub.